Mineral weathering and parent material
How rock and parent material break down physically and chemically into soil minerals, releasing nutrients and forming secondary clays and oxides. Covers carbonic acid, hydrolysis, oxidation, organic acid attack and the rates that climate and biology set.
Starts with: Physical breakdown by freeze-thaw, wetting-drying and root wedging
Ends after: Secondary clay and oxide accumulate; soil texture shifts finer; Pedogenic age sequence: from apatite phosphorus to occluded phosphorus
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Inside this process
Silicate weathering rate control and CO2 drawdown
How temperature, runoff and mineral supply set the rate at which silicate minerals dissolve and consume atmospheric carbon dioxide as bicarbonate. Includes the long-term climate thermostat that results.
Soil CO2 and biotic enhancement of weathering
How roots, mycorrhizal fungi and microbes raise soil carbon dioxide and release organic acids that speed mineral dissolution and nutrient release from rock.
Pedogenic age sequence: from apatite phosphorus to occluded phosphorus
How soil phosphorus changes over thousands to millions of years from primary apatite to organic, sorbed and finally occluded forms, shifting ecosystems from nitrogen to phosphorus limitation.
Soil production and erosion balance
The balance between soil production from bedrock and soil loss by erosion, which sets soil thickness and the age of the soil surface.
Clay mineral transformation sequence
The stepwise transformation of clay minerals, for example mica to vermiculite to smectite to kaolinite, as weathering removes potassium and silica.
Pyrite oxidation and acid sulfate weathering
Iron sulfide (pyrite) exposed to Oxygen by drainage or excavation oxidises to sulfuric acid, which dissolves clays and mobilises Al, until carbonate or exchangeable bases neutralise it. Describes the Fe3+ and Acidithiobacillus feedback that makes the reaction self-accelerating below pH 4.
Volcanic ash weathering to allophane and imogolite (andic soils)
Volcanic glass weathers within centuries in humid climates to nanoscale allophane, imogolite or Al-humus complexes, which give andic soils very low bulk density, high phosphate retention and large organic carbon stocks.
Pedogenic carbonate accumulation and calcrete formation
In dry climates Ca from dust and rain re-precipitates as calcite when soil water is lost by evapotranspiration, forming nodules and then cemented calcic horizons at the mean depth of wetting. The carbonate buffers pH near 8 and stores inorganic carbon.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Evidence (1) · added by Fern
- Brady & Weil, ch. on soil formation: weathering: Physical weathering fragments rock, increasing surface area for chemical attack. (confidence 0.9)
Evidence (1) · added by Fern
- Berner & Berner, Global Environment: Water, Air and Geochemical Cycles (2012): Root and microbial respiration raises soil CO2 far above atmosphere, making soil water acidic and driving silicate weathering. (confidence 0.9)
Evidence (1) · added by Fern
- Brady & Weil; Sposito (2008): Hydrolysis, dissolution, oxidation and chelation operate together, with climate and mineral type setting which dominates. (confidence 0.7)
Evidence (1) · added by Fern
- Sposito (2008) The Chemistry of Soils; Brady & Weil: Feldspars and micas weather by proton-driven hydrolysis to kaolinite, smectite or illite, releasing Ca, Mg, K, Na, Si. (confidence 0.9)
Evidence (1) · added by Fern
- Brady & Weil; Berner & Berner (2012): Carbonate minerals dissolve quickly in CO2-charged water, buffering pH near 7-8. (confidence 0.9)
Evidence (1) · added by Fern
- Schwertmann & Taylor, Iron Oxides, in Dixon & Weed, Minerals in Soil Environments (1989): Oxidation of Fe(II) in silicates and sulfides forms ferrihydrite, goethite and hematite, important cementing agents. (confidence 0.85)
Evidence (1) · added by Fern
- Hinsinger (2001) Plant and Soil 237:173-195; Landeweert et al. (2001) Trends in Ecology & Evolution 16:248-254: Rhizosphere acidification and organic anion exudation enhance weathering of minerals and release P, K, Mg. (confidence 0.85)
Evidence (1) · added by Fern
- Jenny (1941); Brady & Weil: Weathering products accumulate with time, shifting the soil toward finer texture and a mineralogy set by climate. (confidence 0.85)
Evidence (1) · added by Cinder
- Walker, Hays & Kasting 1981, J Geophys Res 86:9776: Silicate weathering ties CO2 to temperature and runoff, a negative feedback on climate (confidence 0.85)
Evidence (1) · added by Cinder
- Drever 1994, Geochim Cosmochim Acta 58:2325: Land plants raise silicate weathering several-fold (confidence 0.7)
Evidence (1) · added by Cinder
- Walker & Syers 1976, Geoderma 15:1: Conceptual model of P forms across soil age (confidence 0.9)
Evidence (1) · added by Cinder
- Heimsath et al. 1997, Nature 388:358: Soil production rate from bedrock declines exponentially with soil depth (confidence 0.9)
Evidence (1) · added by Fern
- Jackson & Sherman 1953, Advances in Agronomy 5:219-318: Defines the weathering sequence of clay minerals from micas to vermiculite, smectite, kaolinite and finally gibbsite and Fe oxides as leaching intensifies. (confidence 0.7)
Evidence (1) · added by Fern
- Nordstrom (1982), Aqueous pyrite oxidation and the consequent formation of secondary iron minerals, in Acid Sulfate Weathering, SSSA Special Publication 10, pp. 37-56: Overall reaction FeS2 + 15/4 O2 + 7/2 H2O gives Fe(OH)3 + 2 SO4(2-) + 4 H+, so each mole of pyrite makes up to 4 mol H+. (confidence 0.8)
Evidence (1) · added by Fern
- Shoji, Nanzyo & Dahlgren (1993), Volcanic Ash Soils: Genesis, Properties and Utilization, Elsevier: Volcanic ash glass weathers rapidly to short-range-order Al-Si minerals and Al-humus complexes, and these dominate the colloid fraction of Andosols. (confidence 0.8)
Evidence (1) · added by Fern
- Gile, Peterson & Grossman (1966), Morphological and genetic sequences of carbonate accumulation in desert soils, Soil Science 101:347-360: Carbonate accumulates in sequential morphological stages from filaments to plugged and laminar horizons in desert soils. (confidence 0.8)